The M2 Bradley is the US Army's first mechanized infantry combat vehicle. The first production models appeared in 1981 (at the height of the Cold War), and they were soon being produced at the rate of 600 per year. The hull of the M2 is made of aluminum, with a layer of space laminated armor applied for added protection. The Bradley is equipped with a 25mm Bushmaster cannon and stabilizer to allow for firing on the move. Its troop compartment, located in the rear, is fitted with firing ports and periscopes to allow troops to fire from within the vehicle. Night vision capability and a nuclear, biological and chemical (NBC) defense system are standard on all of the different variants. The Bradley plays a key role in the US Army's combined arms concept, but critics say it is too big, too expensive, and too difficult to maintain and is insufficiently armored to operate with main battle tanks on the battlefield.